This invention is in the technical field of fuel cell. More particularly, it is in the technical field of manufacturing fuel cell components, and fabrication of fuel cell components incorporating vertically free-standing graphene-containing carbon nanosheets.
A fuel cell is a device that electrochemically converts energy from a fuel into electricity either through reducing positively charged ions (e.g. protons, H+) or via oxidizing a fuel (e.g. hydrogen gas, H2) agent. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by their operating temperature and the type of electrolyte they use, e.g. proton exchange membrane fuel cell (PEMFC), solid oxide fuel cell (SOFC), alkaline fuel cell (AFC), direct-methanol fuel cell (DMFC), molten-carbonate fuel cell (MCFC), and phosphoric-acid fuel cell (PAFC).
A simplified diagram of a PEMFC is shown in FIG. 1, whose components consists of, but not limited to, an anode 110, a cathode 130, and an electrolyte 120 that allows charged ions to move between cathode and anode. In a PEMFC, the anode and cathode contain catalyst 114 and 134, where the reactions occur. Reactant channel plate 111 conducts fuels (e.g. H2) into the anode. Reactant channel plate 131 conducts oxidizing agent (e.g. air or O2) into the cathode.
In most types of PEMFC, current collectors 112 and 132 are attached to the inner side or outer side of reactant channel plate to collect electrical current generated by the fuel cell.
In some types of fuel cells, e.g. PEMFC, gas diffusion layers (GDL) 113 and 133 are inserted between current collector and catalyst layer to electrically connect the catalyst and current collector.
As one kind of thin film material, a carbon nanosheet is a novel carbon nanomaterial with a graphene and graphitic structure developed by Dr. J. J. Wang et al. at the College of William and Mary. As used herein, a “carbon nanosheet” refers to a carbon nanomaterial with a thickness of two nanometers or less. A carbon nanosheet is a two-dimensional graphitic sheet made up of a single to several layers of graphene. Thus, thickness of a carbon nanosheet can vary from a single graphene layer to multiple layers, such as one to seven layers of graphene. For example, a carbon nanosheet may comprise one to three graphene layers and has thickness of one nanometer or less. Edges of a carbon nanosheet usually terminate by a single layer of graphene. The specific surface area of a carbon nanosheet is between 1000 m2/g to 2600 m2/g. The height of a carbon nanosheet varies from 100 nm to 20 μm, depending on fabrication conditions. The width of a carbon nanosheet also varies from hundreds of nanometers to a few microns.
A plurality of carbon nanosheets, each of which comprises at least one layer of graphene, are disposed orthogonally to a coated surface of a substrate. Essentially, the plurality of vertically free-standing carbon nanosheets are functioning as space-organizers at nanoscale. By partitioning the space above the surface of the substrate, these vertically free-standing carbon nanosheets can greatly enlarge the surface area of the substrate.
Hereby the term “free-standing” or the term “vertically free-standing” refers to attaching carbon nanostructures to a surface orthogonally, or at various angles from 0 to 180 degree with respect to the surface. Furthermore, carbon nanostructures stretch out not only in a straight way, but also can have a crumpling, tilting, folding, sloping, or “origami”-like structure.
By virtue of their graphene and graphitic structure, carbon nanosheets have very high electrical conductivity. Graphene is known as one of the strongest materials, and it has a breaking strength over 100 times greater than that of a hypothetical steel film of the same thickness. Morphology of carbon nanosheets can remain stable at temperatures up to 1000° C. A carbon nanosheet has a large specific surface area because of its sub-nanometer thickness. Referring to FIG. 4, it shows an exemplary carbon nanosheet consisting of one layer of graphene. With only 1 to 7 layers of graphene, the carbon nanosheet is about 1 nm thick. Its height and length is about 1 micrometer respectively. The structure and fabrication method of carbon nanosheets have been published in several peer-reviewed journals such as: Wang, J. J. et al., “Free-standing Subnanometer Graphite Sheets”, Applied Physics Letters 85, 1265-1267 (2004); Wang, J. et al., “Synthesis of Carbon Nanosheets by Inductively Coupled Radio-frequency Plasma Enhanced Chemical Vapor Deposition”, Carbon 42, 2867-72 (2004); Wang, J. et al., “Synthesis and Field-emission Testing of Carbon Nanoflake Edge Emitters”, Journal of Vacuum Science & Technology B 22, 1269-72 (2004); French, B. L., Wang, J. J., Zhu, M. Y. & Holloway, B. C., “Structural Characterization of Carbon Nanosheets via X-ray Scattering”, Journal of Applied Physics 97, 114317-1-8 (2005); Zhu, M. Y. et al., “A mechanism for carbon nanosheet formation”, Carbon, 2007.06.017; Zhao, X. et al., “Thermal Desorption of Hydrogen from Carbon Nanosheets”, Journal of Chemical Physics 124, 194704 (2006), as well as described by Zhao, X. in U.S. Patent “Supercapacitor using carbon nanosheets as electrode” (U.S. Pat. No. 7,852,612 B2); and Wang, J. et al., in U.S. Patent “Carbon nanostructures and methods of making and using the same” (U.S. Pat. No. 8,153,240 B2), which are incorporated herein by reference in their entirety.